1. Field of the Invention
The present invention relates to liquid immersion photolithography, and more particularly, to a method and system for confining liquid flow in an immersion photolithographic system.
2. Description of the Related Art
Optical lithography, using lens systems and catadioptric systems, is used extensively in the semiconductor manufacturing industry for the printing of circuit patterns. To date, the gap between a final lens element and a semiconductor wafer surface has been filled with gas, usually air or nitrogen. This gaseous gap works well particularly when the wafer is scanned under the optics during exposure and there is relative movement between the wafer and the lens system during the image transfer.
The practical limits of optical lithography assume that the medium through which imaging is occurring is air. This practical limit is defined by the equation
  Λ  =      λ          4      ·      n      ·      NA      , where 8 is the wavelength of incident light, NA is numerical aperture of the projection optical system, and n is the index of refraction of the medium (where 4 is used instead of 2 due to the use of off axis illumination). The gas interface between the final lens element and the wafer surface limits the maximum resolution of the optical system to a numerical aperture of <1.0. If the gas space between the final lens element and the wafer surface can be filled with a refractive material, such as oil or water, then the numerical aperture, and hence the resolution capability, of the system can be significantly increased, corresponding to the index of refraction n.
Thus, by introducing a liquid between a last lens element of the projection optical system and a wafer being imaged, the refractive index changes, thereby enabling enhanced resolution with a lower effective wavelength of the light source. Immersion lithography effectively lowers a 157 nm light source to a 115 nm wavelength (for example, for n=1.365), enabling the printing of critical layers with the same photolithographic tools that the industry is accustomed to using today.
Similarly, immersion lithography can push 193 nm lithography down to, for example, 145 nm (for n=1.33). 435 nm, 405 nm, 365 nm, 248 nm, 193 nm and 157 nm tools can all be used to achieve effectively better resolution and “extend” the usable wavelengths. Also, large amounts of CaF2, hard pellicles, a nitrogen purge, etc.—can be avoided. Also, depth of focus can be increased by the use of liquid immersion, which may be useful, for example, for LCD panel manufacturing.
However, despite the promise of immersion photolithography, a number of problems remain, which have so far precluded commercialization of immersion photolithographic systems. One problem of existing immersion photolithographic systems involves the difficulties of confining the liquid that is used in an interface between the projection optical system and the wafer being exposed. In conventional systems, liquid is injected between the projection optical system and the wafer. Fairly complex systems have been proposed in order to maintain the confinement of the liquid.
An additional problem exists where the scanning motion of the wafer is such that the wafer is moved away from the exposure area, resulting in a spilling of the liquid. Such spillage is also a problem even when the wafer is present under the projection optical system due to the inherent viscosity properties of the liquid.
Accordingly, what is needed is a simple system and method for confining the liquid between the projection optical system and the wafer.